In a gas turbine, the compressor section produces compressed air that is subsequently heated by burning fuel in a combustion section. The hot gas from the combustion section is directed to a turbine section where the hot gas is used to drive a rotor shaft for producing power in a known manner. The combustion section is typically comprised of a shell, or cylinder casing, that forms a chamber for receiving compressed air from the compressor section. A plurality of cylindrical combustors are disposed within the chamber and receive the compressed air, along with the fuel to be burned. A duct is connected to the aft end of each combustor and serves to direct the hot gas from the combustors to the turbine section.
Conventional industrial gas turbines that have dual fuel capability, i.e., burn natural gas fuel and liquid fuel, have diffusion type burners in the combustors which require only one supply pipe for gas and one supply pipe for liquid fuel. Accordingly, in order to mount the combustor and the accompanying fuel supply lines and nozzles on the turbine cylinder casing, it has previously only been necessary to provide a relatively small aperture in the cylinder casing. In such conventional gas turbines, the burner nozzles are bolted onto the outside of the cylinder casing and the combustor flange is mounted to the inner wall of the casing. Accordingly, the burner nozzles extend through the aperture in the casing and the necessary fuel supply lines are connected to the nozzles outside of the casing.
In order to control environmental pollution caused by gas turbines, it is necessary to reduce the levels of NOx emissions caused by the burning of fuel at high temperatures. In the conventional gas turbines described above, NOx emissions have been controlled by injecting steam and water into the combustors to reduce the temperature at which the fuels are burned. However, these systems are complex and require the additional components necessary to provide steam to the combustors, thus increasing the cost and complexity of the system. Moreover, the known steam injection systems have not provided the desired reductions in pollution levels.
In order to obviate the pollution problems caused by known conventional turbines and avoid the increased cost and complexity associated with steam injection systems, turbine systems have been developed which include newly designed low NOx combustors. These low NOx combustors provide for reduced pollution levels by operation of the combustors in a premix operation that is known in the art, rather than the diffusion burn operation of conventional turbines. Thus, all of the components necessary for the control of NOx emissions are contained within these new low NOx combustors.
However, known low NOx combustors are significantly larger than conventional combustors. Also, these low NOx combustors typically require a pilot nozzle and a two stage main nozzle, thus requiring six fuel supply lines for each combustor, three lines for gas and three lines for liquid fuel. Therefore, as compared to conventional combustor systems, these low NOx combustors systems are significantly larger and comprise more structure that muse be installed into the turbine cylinder. Although new gas turbine power stations can be built with low NOx combustors by designing the new cylinder casing to account for the increased size and complexity of the low NOx combustor apparatus, it is not possible to directly install these larger combustor systems in the relatively small existing aperture of the cylinder casing of conventional turbines. Moreover, it is not feasible to alter the size of the existing cylinder aperture to account for the larger size of the low NOx combustors.
Thus, there is need for apparatus for retro-fitting existing gas turbines easily and at low cost so that low NOx combustors, along with the necessary fuel supply and nozzle structures, can be installed using the existing openings in the cylinder casing and without significant alteration of the casing. The present invention provides a low NOx combustor retro-fit system which satisfies that need.